In particular, it’s not clear how Kerry’s job will mesh with the activities of his former deputy, incoming Secretary of State Antony Blinken — who now technically outranks his old boss — and what role he’ll play on the National Security Council, where he’s expected to attend every top meeting and will likely enjoy direct access to Biden, his longtime Senate colleague and peer.

Kerry’s large personality and “maniacal” pursuit of diplomatic wins, as two former colleagues described it, has consequently raised questions in Biden’s orbit about whether the 76-year-old statesman might overshadow other key figures, including Blinken, who has a quieter and more understated temperament, according to those who know him.

And while Kerry is known as a grinder who bores in on whatever mission he’s given — be it the Iran nuclear deal or diplomatic talks over Syria — his reputation for “diplomatic adventuring,” as one 2013 Atlantic profile put it, could make for an awkward dynamic, at least at first. Kerry, who last reported directly to the president, will soon find himself working out of the State Department again—but this time, as a subordinate to Blinken, who is “closer to Biden on foreign policy than anyone else,” as a former Obama NSC official put it. Kerry will also have a domestic counterpart focusing specifically on climate issues in the United States with whom he will be expected to coordinate fairly closely.